fix(errata): check for SCU before accessing DSU

The DSU contains system control registers in the SCU and L3 logic to
control the functionality of the cluster. If "DIRECT CONNECT" L3 memory
system variant is used, there won't be any L3 cache, snoop filter, and
SCU logic present hence no system control register will be present.
Hence check SCU presence before accessing DSU register for DSU_2313941
errata.

(commit message taken from commit
942013e1dd by Pramod Kumar
<pramod.kumar@broadcom.com> just errata number changed)

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Change-Id: I38cee6085d6e49ba23de95b3de08bc98798ab2b3

lib/cpus/aarch64/dsu_helpers.S

@@ -151,13 +151,22 @@ endfunc errata_dsu_936184_wa
 	 * This function is called from both assembly and C environment. So it
 	 * follows AAPCS.
 	 *
-	 * Clobbers: x0-x3
+	 * Clobbers: x0-x4
 	 * -----------------------------------------------------------------------
 	 */
 	.globl	check_errata_dsu_2313941
 	.globl	errata_dsu_2313941_wa
 
 func check_errata_dsu_2313941
+	mov	x4, x30
+	bl	is_scu_present_in_dsu
+	cmp	x0, xzr
+	/* Default error status */
+	mov	x0, #ERRATA_NOT_APPLIES
+
+	/* If SCU is not present, return without applying patch */
+	b.eq	1f
+
 	mov	x2, #ERRATA_APPLIES
 	mov	x3, #ERRATA_NOT_APPLIES
 
@@ -170,7 +179,8 @@ func check_errata_dsu_2313941
 	mov	x1, #(0x31 << CLUSTERIDR_REV_SHIFT)
 	cmp	x0, x1
 	csel	x0, x2, x3, LS
-	ret
+1:
+	ret	x4
 endfunc check_errata_dsu_2313941
 
 	/* --------------------------------------------------